Bulk acoustic wave resonator and circuit comprising same

ABSTRACT

The invention relates to a resonator operating with bulk acoustic waves (BAW resonator, BAW=Bulk Acoustic Wave) and band-pass filters constructed of such resonators. To increase the edge steepness of the transmission band of a BAW band-pass filter, the invention proposes reducing the effective coupling of a BAW resonator by using the connection in parallel of a BAW resonator and a capacitor instead of only one resonator. In addition, to increase the edge steepness of the transmission band, the use of a connection of coupled BAW resonators in the serial branch of a filter circuit with another resonator or resonator stack in the parallel branch of the filter circuit is proposed, the additional resonator or resonator stack being connected to the center electrode of the resonator stack specified initially.

The invention relates to a resonator operating with bulk acoustic waves(or FBAR, Thin Film Bulk Acoustic Wave Resonator), also known as BAWresonator (Bulk Acoustic Wave Resonator), as well as a circuitconstructed of such resonators.

BAW resonators are suitable, in particular, for band-pass high-frequencyfilters in modern filter technology, and can be used, for example, inmobile communication devices.

A resonator operating with bulk acoustic waves has a piezoelectric layerthat is disposed between two metal layers (electrodes). It is known thata sequence of layers can also be used instead of only one piezoelectriclayer. The layers are deposited consecutively on a substrate andstructured into resonators, which are electrically connected to oneanother and together can constitute a filter circuit, for example,especially a band-pass filter. Such a band-pass filter can also be usedtogether with another filter in a duplexer.

FIG. 1 shows the equivalent circuit diagram of a BAW resonator. Outsidea frequency range surrounding the resonant frequency, the resonator ischaracterized by a static capacitor C₀ and, in proximity to the resonantfrequency, by the connection in series of a resistor R_(m), a capacitorC_(m) and an inductive resistor L_(m). The static capacitor isessentially defined by the resonator surface area and the thickness ofthe piezoelectric layer. The resistor R_(m) describes losses in theresonator, while the capacitor C_(m) and the inductive resistor L_(m)determine the resonant frequency$f_{r} = {\frac{1}{2\quad\pi\sqrt{L_{m}C_{m}}}.}$The ratio C_(m)/C_(o) determines the coupling of the resonator. Thecoupling coefficient k of the resonator is linked to the resonantfrequency f_(r) and the antiresonant frequency f_(a):${k^{2} = \frac{f_{a}^{2} - f_{r}^{2}}{f_{a}^{2}}},$

wherein f_(a)=f_(r)√{square root over (1+C_(m)/C_(o))}.

A band-pass filter is characterized by a transfer function that has, inparticular, a transmission band and several stop bands. The transmissionband is, in turn, characterized by its bandwidth, the insertionattenuation in the transmission band and the edge steepness at the edgeof the transmission band.

It is known that two BAW resonators SR1 and SR2 (as depictedschematically in FIG. 2) can be acoustically coupled with one anotherif, for example, they are arranged in a stack on top of one another. Inthis connection, said resonators form a series connection between a portP1 and a port P2, e.g., in a stacked-crystal arrangement, in which tworesonators share a common electrode, which is connected to ground (seeFIG. 3), or are arranged in a coupled-resonator arrangement, in which acoupling layer KS is arranged between the upper electrode E2 of thelower resonator and the lower electrode E3 of the upper resonator, andsaid electrodes are connected to ground (see FIG. 4). A first resonatorin FIG. 3 comprises a piezoelectric layer PS1, which is arranged betweentwo electrodes E1 and E2, and an acoustic mirror AS arranged below theelectrode E1, said acoustic mirror resting on a carrier substrate TS.Above the first resonator, a second resonator is arranged that comprisesa piezoelectric layer PS2, which is arranged between the electrode E2and an electrode E3. Electrode E1 is connected to port P1, electrode E3to port P2 and electrode E2 to ground.

The layer system shown in FIG. 4 consists of a first resonator arrangedon a carrier substrate TS, a coupling layer KS disposed above it and asecond resonator arranged above the coupling layer KS. The firstresonator is arranged as described in FIG. 3, and is connected betweenport P1 and ground. The second resonator contains (from bottom to top)two electrodes E3 and E4 and a piezoelectric layer PS2 arranged betweensaid electrodes, the second resonator being connected between port P2and ground. The coupling layer KS arranged between said resonatorsprovides for acoustic coupling between these resonators.

Filters constructed of acoustically coupled resonators are characterizedby a high stop band suppression. However, the edge steepness and, withit, the adjacent channel selectivity are comparatively low, due to theabsence of defined pole positions in proximity to the pass band.

It is known that BAW resonators can be connected in a ladder-type or alattice-type construction. The advantage of the lattice-type arrangementof the resonators in a band-pass filter is that the selection of such afilter in stop band areas well outside the transmission band is verygood, ranging, for example, between −40 and −60 dB. The disadvantage ofsaid filter arrangement consists in a low edge steepness of thetransmission band. For this reason, it is difficult, in this type offilter arrangement, to achieve sufficient attenuation of the signal inthe stop band in proximity to the transmission band.

Considerable edge steepness is required in some applications. In thecase of duplexers that are suitable for the PCS telecommunicationsstandard, for example, a decline in the transmission function from ca.−3 dB to significantly below −40 dB within a frequency range of only 20MHz must be guaranteed. Previously known band-pass filters, which areconstructed of BAW resonators, do not satisfy such requirements, due toadditional frequency shifts in the edges in response to temperaturechange or as a result of existing production tolerances (which, in thecase of a filter operating at ca. 2 GHz and having BAW resonators thatcontain a piezoelectric layer of ALN, can amount to several MHz).

It is known, from the reference EP 0949756 A2, that a series connectionof stacked resonators acoustically coupled with one another, as well asadditional resonators instead of only one resonator in a filter circuit,improves edge steepness in the transmission band of the filter. Thedisadvantage of this solution, however, is that it requires a great dealof space.

The goal of the present invention is to specify a BAW resonator thatguarantees a large edge steepness of a band-pass filter constructed ofsuch resonators.

The underlying goal of the invention is solved, in accordance with theinvention, by a resonator according to claim 1. Advantageous exemplaryembodiments may be derived from further claims.

The invention specifies a resonator operating with bulk acoustic waves(also known as BAW resonator—Bulk Acoustic Wave Resonator—or FBAR—ThinFilm Bulk Acoustic Wave Resonator), which is constructed of a sequenceof layers containing the following layers: a first layer region thatcomprises a first electrode, an upper layer region that comprises asecond electrode and, between the two, a piezoelectric layer. Acapacitor is connected in parallel or in series to said resonator.

The connection in parallel of a BAW resonator and a capacitor C_(a)instead of a non-connected resonator reduces the effective coupling ofthe BAW resonator (that is, the distance between the resonant andantiresonant frequency of the resonator), in that the effective staticcapacitor C′₀ is increased, C′₀=C₀+C₀. In this connection, the resonantfrequency f′_(r) of the new circuit (series resonance, or the resonantfrequency of the serial resonant circuit formed by C_(m), L_(m) andR_(m)) remains unchanged relative to the resonant frequency f_(r) of the(non-connected) resonator, f′_(r)=f_(r). In contrast, the antiresonantfrequency f′_(a)=f_(r)√{square root over (1+C_(m)/C′₀)} (parallelresonance, or the resonant frequency of the parallel resonant circuitformed by C′₀, C_(m), L_(m) and R_(m)) is lower than the antiresonantfrequency f_(a)=f_(r)√{square root over (1+C_(m)/C₀)} (parallelresonance, or the resonant frequency of the parallel resonant circuitformed by C₀, C_(m), L_(m) and R_(m)) of the (non-connected) resonator.As a result, the edge steepness of a band-pass filter comprising suchBAW resonators is increased.

The connection in series of a BAW resonator and a capacitor C_(a)instead of a non-connected resonator reduces the effective coupling of aBAW resonator (that is, the distance between the resonant and theantiresonant frequency of the resonator). In the connection, theantiresonant frequency f′_(a) of the circuit (parallel resonance, or theresonant frequency of the parallel resonant circuit formed by C₀, C_(m),L_(m) and R_(m)) remains unchanged relative to the antiresonantfrequency f_(a) of the resonator, f′_(a)=f_(a). In contrast, theresonant frequency f′_(r)=f_(r)√{square root over (1+C_(m)/(C_(a)+C₀)}(series resonance, or the resonant frequency of the serial resonantcircuit formed by C₀, C_(m), L_(m) and R_(m)) of said circuit is higherthan the resonant frequency f_(r) (series resonance, or the resonantfrequency of the serial resonant circuit formed by C_(m), L_(m) andR_(m)) of the resonator. As a result, the edge steepness of a band-passfilter comprising such BAW resonators is increased.

In an advantageous embodiment of the invention, the resonator of theinvention is arranged on a carrier substrate. It is also possible toarrange the resonator of the invention over an air gap provided in thecarrier substrate.

The first and the second electrode consist of an electrically conductivematerial, such as a metal or a metal alloy.

The piezoelectric layer preferably consists of AlN, but can also consistof another material with piezoelectric properties (such as ZnO). It isalso possible that the piezoelectric layer comprises a plurality ofadjacent or separated, identical or different layers with piezoelectricproperties.

It is possible that the first and/or the second electrode has amultilayer structure consisting of two or more adjacent layers ofdifferent materials. It is also possible that the piezoelectric layer inthe resonator of the invention comprises two or more adjacent orseparated layers of different materials.

It is possible that, additionally, a layer resistant to dielectricdischarge is arranged between the first and the second electrode, saidlayer protecting the resonator against electric arcing between theelectrodes.

The connection, according to the invention, of a capacitor in parallelto a BAW resonator can be accomplished in a filter constructed, forexample, in a ladder-type construction, in a lattice-type constructionor as an SCF (Stacked Crystal Filter), as well as of any combination ofsaid BAW resonators according to the invention.

It is possible to provide for the connection of a capacitor in parallelto a BAW resonator in only one serial branch or in a plurality of serialbranches of a filter. It is also possible to provide for the connectionof a capacitor in parallel to a BAW resonator in only one parallelbranch or a plurality of parallel branches of a filter. In a furtherembodiment, it is possible that the connection of a capacitor inparallel to a BAW resonator be provided in at least one serial branch orin at least one parallel branch of the filter.

In said exemplary embodiments, the value of the capacitor connected,according to the invention, in parallel to a BAW resonator preferablyranges between 0.1 and 10 pF.

It is advantageous when the coupling of the resonator is reduced only inthe serial branches or only in the parallel branches of a filter or aduplexer by the connection in parallel of the corresponding capacitors.

It is possible to implement the capacitor connected in parallel to a BAWresonator by connecting a discrete capacitor in parallel to the BAWresonator. Another possibility is to realize such a capacitor in thecarrier substrate by means of structured metal layers. It is alsopossible to arrange an additional dielectric layer between theelectrodes of the BAW resonator to increase the capacitance of the BAWresonator. This dielectric layer can be arranged between thepiezoelectric layer and one of the electrodes or between twopiezoelectric layers.

The parasitic capacitance of the respective resonator can also bedeliberately selected to be as large as possible, for example byenlarging the electrode surface to improve the edge steepness of thefilter constructed of such resonators. Other realizations of theinvention not cited here are also possible.

It is possible that the lower and/or upper layer region of the resonatoraccording to the invention consists of one or more layers. It is alsopossible that an acoustic mirror is realized in the lower and/or in theupper layer region, said mirror comprising at least two alternatinglayers having different acoustic impedance.

The acoustic mirror consists of alternating layers, each having a highand a low acoustic impedance, each of their layer thicknesses comprisingapproximately a quarter wavelength of the acoustic main mode (relativeto the velocity of expansion of the acoustic wave in the respectivematerial). The acoustic mirror thus provides one and/or a plurality ofboundary surfaces, which, at the resonant frequency of the acousticwave, reflect back into the resonator and prevent the wave from escapingin the direction of the carrier substrate.

In a further advantageous embodiment of the invention, one of the layersof the acoustic mirror can simultaneously constitute one of saidelectrodes.

The use, according to the invention, of a BAW resonator with a capacitorconnected in parallel in the circuit of a band-pass filter increases theedge steepness of the transmission band of the band-pass filter. As aresult, the attenuation of the signal is increased in the stop bands inproximity to the transmission band. This is especially advantageous inthe case of realization of a duplexer circuit having such a band-passfilter.

The underlying goal of the invention is, furthermore, achieved by theembodiment variant of the invention described below.

This embodiment according to the invention specifies an electric circuitcontaining a resonator stack that comprises at least two resonatorsarranged on top of one another and operating with bulk acoustic wavesand at least one additional resonator or resonator stack having BAWresonators. Each of said resonators operating with bulk acoustic wavescomprises a lower electrode, an upper electrode and a piezoelectriclayer arranged between the two. In this connection, the resonatorsarranged on top of one another in the resonator stack form a serialconnection, e.g., in a stacked crystal arrangement (when both resonatorshave a shared electrode) or a coupled resonator arrangement (when acoupling layer is provided between the upper electrode of the lowerresonator and the lower electrode of the upper resonator).

In this connection, the upper electrode of the lower resonator operatingwith bulk acoustic waves and the lower electrode of the upper resonatoroperating with bulk acoustic waves, which are arranged in the resonatorstack, is electrically connected with one of the electrodes of at leastone additional resonator or resonator stack.

The connection according to the invention can be viewed as a basicelement of a ladder-type arrangement or (in the case of a suitableconnection) of a lattice-type arrangement of individual resonators, atleast two of said resonators being acoustically coupled with one anotherand arranged on top of one another. In this connection, it is possiblethat two BAW resonators arranged on top of one another in a stackrealize two serial resonators or parallel resonators of the ladder-typearrangement or of the lattice-type arrangement. It is also possible thattwo BAW resonators arranged on top of one another in a stack realize oneserial resonator and one parallel resonator of the ladder-typearrangement or the lattice-type arrangement.

A coupling layer is preferably provided between the upper electrode ofthe lower resonator operating with bulk acoustic waves and the lowerelectrode of the upper resonator operating with bulk acoustic waves,which are arranged in the resonator stack.

The at least one additional resonator can, for example, be a resonatorwith bulk acoustic waves, a resonator operating with acoustic surfacewaves, an LC resonator or a resonator stack as specified above.

The second electrode of the at least one additional resonator, which isnot connected to the resonators arranged on top of one another in theresonator stack, can be connected to ground or to a subsequent resonatorand/or to a resonator stack not yet specified.

The circuit according to the invention represents an advantageouscombination of different filter arrangements known in the art, such asthe arrangement of the resonators stacked on top of one another andacoustically coupled with one another, as well as a ladder-typearrangement and/or a lattice-type arrangement. The transfer function ofa filter whose basic elements realize the circuit according to theinvention, as compared with the transfer function of a filterconstructed of resonator stacks known in the art, exhibits significantlysteeper edges in the transmission band of the filter. This results inexceptionally good adjacent channel selectivity of the filter.

The circuit according to the invention, which, for example, consists ofa resonator stack and a resonator electrically connected with it asspecified above, preferably constitutes a basic element of a filter.

It is possible that a plurality of parallel resonators, each of which isarranged in a parallel branch of different basic element electricallyconnected with one another, are acoustically connected with one anotherand/or arranged on top of one another. It is also possible that, insteadof only one resonator being realized in the parallel branch (parallelresonator) of a basic element of the circuit according to the invention,two (preferably coupled with one another) parallel resonators connectedin series or in parallel are realized.

It is also possible that more than only two serial resonators arearranged on top of one another and/or acoustically coupled with oneanother.

The basic elements of the circuit according to the invention describedabove can be combined with one another in any manner.

In the following, the invention is explained in greater detail on thebasis of figures that are schematic and, therefore, not true to scale.

FIG. 1 shows an equivalent circuit diagram of a BAW resonator

FIG. 2 shows the circuit diagram of a resonator stack

FIG. 3 shows a resonator stack with acoustically coupled BAW resonatorsin schematic cross-section (state of the art)

FIG. 4 shows another example of a resonator stack with acousticallycoupled BAW resonators and a coupling layer in schematic cross-section(state of the art)

FIG. 5 a shows an equivalent circuit diagram of a BAW resonator with acapacitor connected in parallel to it, according to the invention

FIG. 5 b shows an equivalent circuit diagram of a BAW resonator with acapacitor connected in series to it, according to the invention

FIG. 6 a shows a basic element of a filter realized in ladder-typeconstruction with a capacitor connected in parallel to a BAW resonatorin the serial branch

FIG. 6 b shows the transfer function of a filter realized in ladder-typeconstruction without and with a capacitor connected in parallel to a BAWresonator in the serial branch

FIG. 7 shows a basic element of a filter realized in ladder-typeconstruction with a capacitor connected in parallel to a BAW resonatorin the parallel branch

FIG. 8 a shows an exemplary embodiment of a filter realized inladder-type construction with capacitors connected in parallel to BAWresonators in the serial branches

FIG. 8 b shows the transfer function of a filter realized inlattice-type construction without and with a capacitor connected inparallel to a BAW resonator in the serial branch

FIG. 9 shows an exemplary embodiment of a filter realized inlattice-type construction with capacitors connected in parallel to BAWresonators in the parallel branches

FIG. 10 shows a connection, according to the invention, of a resonatorstack in the serial branch and of an additional BAW resonator in theparallel branch, in circuit diagram (a) and in schematic cross-section(b), respectively

FIG. 11 shows an advantageous exemplary embodiment of a connection,according to the invention, of a resonator stack and of an additionalBAW resonator in schematic cross-section

FIG. 12 shows a connection, according to the invention, of a resonatorstack in the serial branch and of an additional resonator stack in theparallel branch, in circuit diagram (a) and in schematic cross-section(b), respectively

FIGS. 1 to 4 have already been discussed earlier. FIG. 5 a shows anequivalent circuit diagram of a BAW resonator with a capacitor C_(a)connected in parallel to it. Outside the resonant frequency range, theresonator is characterized by a static capacitor C₀ and, in proximity tothe resonant frequency, by a resistor R_(m), a capacitor C_(m) and aninductive resistor L_(m). The resistor R_(m) describes losses in theresonator, while the capacitor C_(m) and the inductive resistor L_(m)determine the resonant frequency. The ratio C_(m)/C₀ determines thecoupling of the resonator. The addition of a capacitor C_(a) connectedin parallel to the resonator results in reduction of the effectivecoupling of the resonator, which is now determined by C_(m)/(C₀+C_(a)),instead of C_(m)/C₀.

FIG. 5 b shows an equivalent circuit diagram of a BAW resonator with acapacitor C_(a) connected in series to it.

An exemplary connection of two BAW resonators RA and RB in ladder-typeconstruction and a capacitor C_(a) connected in parallel to one of saidresonators is shown in FIG. 6 a. Resonator RA is arranged in a serialbranch and resonator RB in a parallel branch of the circuit. Tworesonators connected in this manner represent, for example, a basicelement of a ladder-type filter known in the art.

In FIG. 6 a, the capacitor C_(a) is integrated in the serial branch ofthe circuit. In this connection, it is connected in parallel to theserial resonator RA, as a result of which the steepness of the rightedge of the transfer function in the transmission band can be controlledand/or increased. Such a basic element can, for example, be used in atransmission filter (Tx filter) of a duplexer, especially a PCSduplexer.

FIG. 6 b shows the transfer function S21 of a filter realized inladder-type construction without and with a capacitor connected inparallel to a BAW resonator in the serial branch. The transfer functionof the filter constructed of BAW resonators in the ladder-typeconstruction known in the art is indicated by a dashed line 11. Thetransfer function of the filter according to the invention inladder-type construction with a capacitor connected in parallel to a BAWresonator in the serial branch is indicated by a continuous line 12,wherein the transfer function, in this case, has a steeper right edge ofthe transmission band.

In FIG. 7, the capacitor C_(a) is integrated in the parallel branch ofthe circuit. In this connection, it is connected in parallel to theparallel resonator RB, as a result of which the steepness of the leftedge of the transfer function in the transmission band can be controlledand/or increased. Such a basic element can, for example, be used in areception filter (Rx filter) of a duplexer, especially a PCS duplexer.

The capacitor C_(a) can be arranged on a carrier substrate, togetherwith the BAW resonator. The capacitor C_(a) can also constitute adiscrete component with external electrodes, which is electricallyconnected with the BAW resonator as described above.

It is also possible that the capacitor C_(a) is realized in themetallized layers of the (multilayer) carrier substrate and, asdescribed above, is electrically connected with the BAW resonator bymeans, for example, of feedthroughs, bump connectors or bond wires.

An example of a connection of two BAW resonators RA and RB inlattice-type construction and a capacitor C_(a) connected in parallel toone of said resonators is shown in FIG. 8 a. A resonator RA is arrangedin a serial branch, and a resonator RB in a parallel branch of thecircuit. FIG. 8 a shows two pairs of resonators connected in thismanner, which, for example, constitute a basic element of a filterrealized in lattice-type construction known in the art.

In FIG. 8 a, each of two capacitors C_(a) is integrated in a serialbranch of the circuit. In this connection, each is connected in parallelto the corresponding serial resonator RA, as a result of which thesteepness of the right edge of the transfer function in the transmissionband can be controlled and/or increased. Such a basic element can, forexample, be used in a transmission filter (Tx filter) of a duplexer,especially a PCS duplexer.

FIG. 8 b shows the transfer function S21 of a filter realized inlattice-type construction without and with a capacitor connected inparallel to a BAW resonator in the serial branch. The transfer functionof the filter constructed of BAW resonators in the lattice-typeconstruction known in the art is indicated by a dashed line 11. Thetransfer function of the filter according to the invention inlattice-type construction with a capacitor connected in parallel to aBAW resonator in the serial branch is indicated by a continuous line 12,wherein the transfer function, in this case, has a steeper right edge ofthe transmission band.

In FIG. 9, each of two capacitors C_(a) is integrated in a parallelbranch of the circuit. In this connection, each is connected in parallelto the parallel resonator RB, as a result of which the steepness of theleft edge of the transfer function in the transmission band can becontrolled and/or increased. Such a basic element can, for example, beused in a reception filter (Rx filter) of a duplexer, especially a PCSduplexer.

FIG. 10 a shows the circuit diagram of a connection, according to theinvention, of a resonator stack, which comprises the BAW resonators SR1and SR2, in the serial branch, and of an additional BAW resonator PR inthe parallel branch. The resonator stack is connected between ports P1and P2. An exemplary realization of such a circuit is shown in schematiccross-section in FIG. 10 b. The resonator stack comprises thepiezoelectric layer PS2, which is arranged between two electrodes E1 andE2 (center electrode). The piezoelectric layer PS2 is arranged abovethem. An electrode E4 connected to the port 2 lies on the piezoelectriclayer PS2. The port P1 is electrically connected with the electrode E1.The layer sequence E1, PS1 and E2 realizes, for example, the resonatorSR1 in accordance with FIG. 10 a. The layer sequence E2, PS2 and E4realizes, for example, the resonator SR2 in accordance with FIG. 10 a.Here, the resonator PR in the parallel branch of the circuit accordingto FIG. 10 a is realized by the layer sequence E6 (electrode), PS3(piezoelectric layer) and E5 (electrode), the electrode E5 beingelectrically connected with the center electrode E2. In this exemplaryembodiment, the electrode E6 is connected to ground. It is also possiblethat it be connected to another circuit not shown here.

FIG. 11 shows, in schematic cross-section, an advantageous embodiment ofthe connection, according to the invention, of a resonator stack and anadditional BAW resonator. The resonator stack consists, from bottom totop, of a first electrode E1, a first piezoelectric layer PS1, a secondelectrode E2, a coupling layer KS1, a third electrode E3, a secondpiezoelectric layer PS2 and a fourth electrode E4. The resonator stackforms two resonators arranged on top of one another and coupled with oneanother by means of the coupling layer (corresponding to SR1 and SR2 inFIG. 10 a), and is connected between ports P1 and P2. The parallelbranch of the circuit is formed by an additional resonator, whichconsists of a third piezoelectric layer PS3 and electrodes E5 and E6surrounding it. Electrodes E2 and E3 are connected with electrode E5.Here, electrode E6 is connected to ground. It is also possible that itbe connected to another circuit not shown here.

FIG. 12 a shows the circuit diagram of a connection, according to theinvention, of a resonator stack in the serial branch and anotherresonator stack in the parallel branch between ports P1 and P2. Thefirst resonator stack consists of two resonators SR1 and SR2 connectedin series. The second resonator stack consists of two resonators PR1 andPR2 connected in series. An exemplary realization of this circuit isshown in schematic cross-section in FIG. 12 b. The first resonator stackis constructed as shown in FIG. 10 b. The second resonator stackconsists, from bottom to top, of an electrode E6 (connected to ground,for example), a piezoelectric layer PS3, a center electrode E5, which iselectrically connected with electrode E2 of the first resonator stack, apiezoelectric layer PS4 and an electrode E7 (connected to ground, forexample).

Though not specifically shown in the figure, the (lower) resonators are,in this case, also arranged on a carrier substrate, an air gap or anacoustic mirror being provided, in each case, between the carriersubstrate and resonator.

In the interest of clarity, the invention was only described on thebasis of a few embodiments, but is not limited to these. Othervariations are possible, especially in light of the possiblecombinations of the basic elements and arrangements presented above, aswell as the number of layers in said layer regions of the resonatoraccording to the invention. The invention is not limited to a specificfrequency range or a specific scope of application. Each of the layersof the resonator according to the invention (e.g., the piezoelectriclayer or the electrode) can have a multilayer structure. The resonatoraccording to the invention can also contain a plurality (e.g., notadjacent to one another) of piezoelectric layers or more than only twoelectrodes.

The electrical connections (including the connections to ground) in theexemplary embodiments described can contain discrete elements, such asinductive resistors, capacitors, delay lines or adjustment networks.

1. An apparatus comprising: a resonator for use with bulk acousticwaves, the resonator comprising: a lower layer region comprising a firstelectrode; an upper layer region comprising a second electrode; and apiezoelectric layer between the first electrode and the secondelectrode; and a capacitor in parallel with the resonator or in serieswith the resonator.
 2. The apparatus of claim 1, further comprising acarrier substrate, the resonator and the capacitor being on the carriersubstrate.
 3. The apparatus of claim 1, wherein each of the upper layerregion and the lower layer region comprises a plurality of layers. 4.The apparatus of claim 3, wherein a plurality of layers in the upperlayer region comprises layers that include different materials, and aplurality of layers in the lower layer region comprises layers thatinclude different materials.
 5. The apparatus of claim 1, wherein atleast one of the upper layer region and the lower layer region comprisesan acoustic mirror, the acoustic mirror comprising at least twoalternating layers having different acoustic impedances.
 6. Theapparatus of claim 5, wherein at least one layer of the acoustic mirrorcomprises an electrode layer.
 7. The apparatus of claim 2, wherein thecarrier substrate includes an air gap; and wherein the resonator is overthe air gap.
 8. A filter comprising resonators for use with bulkacoustic waves, each of the resonators comprising: a lower layer regioncomprising a first electrode; an upper layer region comprising a secondelectrode; and a piezoelectric layer between the first electrode and thesecond electrode; wherein the resonators are in at least one of aladder-type arrangement, a lattice-type arrangement, and a stackedcrystal filter arrangement; and a capacitor in parallel with at leastone or the resonators or in series with at least one of the resonators.9. The filter of claim 8, wherein the capacitor is connected to only oneof the resonators, the capacitor being connected in a circuit path thatis in series with, or in parallel with, the one of the resonators.
 10. Aduplexer comprising a filter according to claim
 8. 11. An electricalcircuit comprising: a stack of resonators comprising: first resonatorsthat operate with bulk acoustic waves, the first resonators comprisingan upper resonator and a lower resonator, each of the upper resonatorand the lower resonator comprising upper and lower electrodes; and asecond resonator the operates with bulk acoustic waves, the secondresonator comprising electrodes; wherein the first resonators and thesecond resonator each comprise: a lower electrode, an upper electrode,and a piezoelectric layer arranged between the upper and lowerelectrodes; and wherein an upper electrode of the lower resonator and alower electrode of the upper resonator are electrically connected to anelectrode of the second resonator.
 12. The electrical circuit of claim11, wherein an electrode of the second resonator is connected to ground.13. The electrical circuit of claim 11, wherein the stack of resonatorsfurther comprises: a coupling layer between the upper electrode of thelower resonator and the lower electrode of the upper resonator.
 14. Theelectrical circuit of claim 11, wherein the second resonator comprisesan LC resonator.
 15. The electrical circuit of claim 11, furthercomprising a second stack of resonators, the second stack of resonatorscontaining the second resonator.
 16. The filter of claim 8, furthercomprising a carrier substrate, at least one of the resonators and thecapacitor being on the carrier substrate.
 17. The filter of claim 8,wherein, for each of the resonators, an upper layer region and a lowerlayer region comprises a plurality of layers.
 18. The filter of claim17, wherein a plurality of layers in each upper layer region compriseslayers that include different materials, and a plurality of layers ineach lower layer region comprises layers that include differentmaterials.
 19. The filter of claim 8, wherein each upper layer regionand each lower layer region comprises an acoustic mirror, each acousticmirror comprising at least two alternating layers having differentacoustic impedances.
 20. The filter of claim 19, wherein at least onelayer of each acoustic mirror comprises an electrode layer.
 21. Thefilter of claim 16, wherein the carrier substrate includes an air gap;and wherein at least one or the resonators is over the air gap.
 22. Aduplexer comprising a filter according to claim 9.